1. Field of the Invention
The present invention relates to a mask for laser irradiation, and more particularly, to a mask for a crystallization process of an amorphous silicon thin film using a sequential lateral solidification, and an apparatus for laser crystallization using the same.
2. Discussion of the Related Art
A sequential lateral solidification (SLS) method is commonly applied to laser crystallization processes, which is one method of crystallizing amorphous silicon thin films into polycrystalline silicon thin films. The SLS method makes use of the fact that silicon grains tend to grow laterally from interfaces between liquid and solid phases of silicon such that grain boundaries are formed perpendicular to the interfaces.
The laser crystallization processes using the SLS method include the follow steps. First, an amorphous silicon thin film is irradiated with a laser beam having a predetermined shape, wherein the laser beam has a sufficient energy to completely melt the amorphous silicon thin film. Then, portions of the amorphous silicon thin film completely melted using the laser beam immediately solidify. After the irradiation step, the amorphous silicon instantaneously comprises a liquid phase within the portions irradiated with the laser beam and a solid phase within other portions not irradiated with the laser beam. Thus, interfaces are generated between the liquid and solid phases. Furthermore, silicon grains laterally grow from the interfaces between the liquid and solid phases during solidification.
Next, after moving the amorphous silicon thin film by a specific distance, the amorphous silicon is irradiated for a second time with the laser beam. Similarly, portions of the amorphous silicon thin film irradiated with the laser beam completely melt, and then the silicon grains laterally grow. Since the silicon grains formed through the first irradiation step function as crystallization seeds at the interfaces, the silicon grains grow along a scanning direction of the laser beam. These process steps are repeated until a desired area of the amorphous silicon thin film is crystallized. Accordingly, grain sizes of the polycrystalline silicon thin film are remarkably enlarged by the SLS method. In addition, the laser crystallization processes using the SLS method includes shaping the laser beam to have a specific width and a specific height. For this purpose, an apparatus for the laser crystallization processes using the SLS method use a mask for shaping the laser beam.
FIG. 1 is a schematic arrangement of an apparatus for laser crystallization processes using an SLS method according to the related art. In FIG. 1, an apparatus for laser crystallization processes using an SLS method includes a laser beam source 10, an attenuator 11, a homogenizer 12, a field lens 13, a laser beam mask 14, an object lens 15, first, second, and third mirrors 19a, 19b, and 19c to adjust a path of the laser beam, and a process chamber 20 having a translation stage 16. An initial laser beam emitted from the laser beam source 10 without treatment is transmitted through the attenuator 11 for adjusting the intensity of the laser beam, and through the homogenizer 12 and the field lens 13 to adjust intensity and uniformity of the laser beam. The laser beam transmitted through the field lens 13 is shaped to have a specific configuration while passing through the laser beam mask 14. Then, the shaped laser beam is transmitted through the object lens 15 for focusing the laser beam, and is irradiated onto a silicon thin film 17 disposed on the translation stage 16. In general, the silicon thin film is formed on a substrate in a liquid crystal display (LCD) device.
FIG. 2A is a schematic plan view of laser beam mask for laser crystallization processes using an SLS method according to the related art, and FIG. 2B is a schematic cross sectional view along II—II of FIG. 2A according to the related art. In FIGS. 2A and 2B, a laser beam shielding pattern 34 including a slit 32 is formed on one surface of a base substrate 30. Accordingly, a laser beam passes through the slit 32 from the other surface of the base substrate 30 during a laser crystallization process.
FIG. 3A is a schematic plan view of a laser beam mask for laser crystallization processes using an SLS method according to the related art, and FIG. 3B is a schematic cross sectional view along III—III of FIG. 3A according to the related art. In FIGS. 3A and 3B, a laser beam shielding pattern 44 including a slit 42 is formed on one surface of a base substrate 40 and an anti-reflecting layer 46 is formed on the other surface of the base substrate 40. Accordingly, a laser beam passes through the slit 42 from the other surface of the base substrate 40 during a laser crystallization process. Thus, reflection of the laser beam at the other surface of the base substrate 40 is minimized by the anti-reflecting layer 46.
In general, the laser beam shielding patterns 34 and 44 (in FIGS. 2A, 2B, 3A, and 3B) are each made of an opaque metallic material, such as chromium (Cr) and aluminum (Al). Furthermore, the anti-reflecting layer 46 (in FIGS. 3A and 3B) reduces reflectance of the laser beam and is formed through a coating method using an organic material as a target. During a laser crystallization process using the laser beam masks of FIGS. 2A, 2B, 3A, and 3B, some of the laser beam is transmitted through the slit 32 and 42 (in FIGS. 2A, 2B, 3A, and 3B) and other portions of the laser beam are absorbed into the laser beam shielding pattern 34 and 44 (in FIGS. 2A, 2B, 3A, and 3B). Accordingly, the absorption of the laser beam may cause some damage to the laser beam shielding pattern 34 and 44 (in FIGS. 2A, 2B, 3A, and 3B). For example, the opaque metallic material of the laser beam shielding pattern 34 and 44 (in FIGS. 2A, 2B, 3A, and 3B) may be thermally oxidized due to high intensity of the laser beam. When the thermal oxidization of the opaque metallic material is severe, the opaque metallic material may be converted and removed into particles. These particles may be attached within the slit 32 and 42 (in FIGS. 2A, 2B, 3A, and 3B), thereby reducing the intensity and/or the uniformity of the laser beam. Accordingly, frequent maintenance, such as cleaning, of the laser beam mask and changing of gases for the laser source is necessary, thereby reducing process yield. Moreover, as intensity and repetition rate of the laser beam increases, the damage of the laser beam mask and reduction of the process yield increases.